Purification of alpha, beta-unsaturated aliphatic aldehydes



United States Patent 2,991,233 PURHICATION 0F ALPHA,BETA-UNSATURATED ALIPHATIC ALDEHYDES Harry de V. Finch, Berkeley, Calif., assignor to Shell Oil Company, New York, N. a corporation of Delaware No Drawing. Filed July 24, 1959, Ser. No. 829,212 13 Claims. (Cl. 202-57) This invention relates to the purification of alpha,betaunsaturated aliphatic aldehydes, and relates more particularly to the removal of contaminating amounts of saturated aliphatic aldehydes from alpha,beta-unsaturated aliphatic aldehydes having closely approximating boiling temperatures. A particular aspect of the invention relates to the separation of contaminating amounts of propionaldehyde from acrolein.

Alpha,beta-unsaturated aliphatic aldehydes as obtained commercially generally contain impurities which include one or more saturated aliphatic aldehydes. Often these contaminant saturated aldehydes have boiling temperatures approximating those of the unsaturated aldehydes containing them. Acrolein often contains in admixture therewith acetaldehyde and/or propionaldehyde The removal to at least a substantial degree of the contaminating saturated aldehydes from the alpha,beta-unsaturated aldehydes by ordinary practical scale distillation means is often not feasible. The ease with which acrolein reacts renders the removal of saturated aldehydes from this particular unsaturated aldehyde especially difficult. Though processes have been disclosed heretofore directed to the purification of these materials, these are not always practical.

It is an object of the present invention to provide an improved process enabling the more eflicient removal of contaminating amounts of saturated aliphatic aldehydes from alpha,beta-unsaturated aliphatic aldehydes. Another object of this invention is the provision of an improved process enabling the more efiicient removal of contaminating amounts of saturated aldehydes from alpha,beta-unsaturated aldehydes containing from 3 to 4 carbon atoms to the molecule and having boiling temperatures closely approximating those of the contaminant saturated aldehydes. Still another object of the invention is the provision of an improved process enabling the more efficient purification of acrolein containing impurities comprising acetaldehyde and propionaldehyde in contaminating amounts. A particular object of the invention is the provision of an improved process enabling the more efficient removal from acrolein of contaminating amounts of propionaldehyde. Other objects and advantages of the present invention will become apparent from the following detailed description thereof.

In accordance with the present invention alpha,betaunsaturated aliphatic aldehydes are freed of saturated aliphatic aldehydes contained in contaminating amounts therein, by treating the saturated aldehyde-containing alpha,beta-unsaturated aliphatic aldehyde with a catalytic amount of aluminum alcoholate at a temperature below about 50 C., and distilling acrolein substantially free of saturated aliphatic aldehydes from the resulting treated mixture.

It is known that aluminum alcoholates will undergo reaction with alpha,beta-unsaturated aldehydes leading to the formation of reaction products comprising selfcondensation products of the unsaturated aldehyde. The present invention is based upon the discovery that in the presence of no more than a catalytic amount of aluminum alcoholates, saturated aliphatic aldehydes present in contaminating amounts in alpha,betaunsaturated aliphatic aldehydes will, under substantially anhydrous con ditions, at a temperature below about 50 C., react with themselves, and to some extent with a portion of the unsaturated aldehyde, at a substantially faster rate than the rate of the reactions involving primarily unsaturated aliphatic aldehydes as the reactants. The addition under these conditions of aluminum alcoholate to'a saturated aldehyde-containing alpha,beta-unsaturated aldehyde in only catalytic amounts will therefore result in a substantially selective reaction of the contaminant saturated aldehyde to form higher boiling reaction products from which the alpha,beta-unsaturated aldehyde is readily separated.

Alpha,beta-unsaturated aldehydes treated in accordance with the invention comprise alpha,beta-unsaturated aliphatic aldehydes from any source containing saturated aliphatic aldehydes therein in contaminating amounts, In addition to the saturated aliphatic aldehydes the impure unsaturated aldehydes treated may contain amounts of carbonylic impurities, such as aliphatic ketones, often encountered therein. Such impurities do not materially affect the process of the invention. They may be removed before or after the treatment by conventional means. The process of the invention is applied with particular advantage to the purification of acrolein and methacrolein containing contaminating amounts of saturated aliphatic aldehydes having boiling temperatures closely approximating those of the acrolein and methacrolein, respectively. The process of the invention enables the efiicient removal of contaminating amounts of acetaldehyde and/ or propionaldehyde from acrolein. The contaminated acrolein thus treated may contain in addition to the saturated aldehydes, minor amounts of such impurities as dimethyl ketone, alkyl alcohol, propylene oxide, etc. In view of the difficulty with which propionaldehyde is removed from acrolein by conventional means as a consequence of their close boiling temperatures, the process of the invention is of particular value in freeing acrolein of the propionaldehyde often contained therein in contaminating amounts. As a further specific example of saturated aldehyde-containing alpha,beta-unsatu rated aldehydes purified in accordance with the invention, mention is made of methacrolein containing impurities comprising isobutyraldehyde and/ or propionaldehyde in contaminating amounts.

The extent to which the saturated aldehyde may be present as a contaminant in the alpha,beta-unsaturated aliphatic aldehydes treated may vary within the scope of the invention. In general, the invention is applied with advantage to the treatment of alpha,beta-unsaturated aldehydes wherein the saturated aldehyde impurity is present in an amount up to, for example, about 10%. Alpha,beta-unsaturated aliphatic aldehydes containing somewhat larger amounts of the contaminating saturated aldehyde may, however, be treated in accordance with the invention. A particular advantage of the invention resides in the efficiency with which traces of contaminating saturated aldehydes are eliminated from the alpha,beta-unsaturated aldehydes containing them. The invention enables the efficient substantially complete removal of the last traces of the undesired saturated aldehyde from alpha,beta-unsaturated aldehydes containing the contaminant in amounts below about 5% and less.

The catalysts employed in the process of the invention are the aluminum alcoholates. Suitable aluminum alcoholates comprise the aluminum alcoholates of alcohols having from one to ten carbon atoms. The suitable aluminum alcoholates comprise those derived from primary, secondary or tertiary aliphatic alcohols and from aromatic alcohols. Examples of such suitable aluminum alcoholates are: aluminum methoxide, aluminum ethoxide, aluminum propoxide, aluminum isopropoxide, aluminum isobutoxide, aluminum allyloxide, aluminum salt of 4-methylpentanol-2, aluminum cyclohexyloxide, etc. The suitable aluminum alcoholates comprise the mixed aluminum alcoholates, that is those wherein the oxygen-containing organic radicals attached to the aluminum atom are dissimilar. Preferred are the aluminum alkoxides wherein each alkoxide group contains from three to eight carbon atoms. Particularly preferred are the aluminum secondary 'alkoxides having from three to four carbon atoms in each of the alkoxide groups. Of these, aluminum secondary butoxide is preferred. Mixtures of two or more of the aluminum alkoxides may be used as the catalyst within the scope of the invention. The suitable aluminum alcoholates are prepared in the manner disclosed in the prior art. They may be prepared, for example, by reacting aluminum with the appropriate alcohol according to the known methods, and if desired in the presence of a promoter or catalyst for the reaction, such as mercuric chloride. A suitable method for preparing aluminum alcoholates is that disclosed in US. Patent 2,845,447 issued July 29, 1958.

In the process of the invention the aluminum alcoholates are employed in no more than catalytic amounts. This is essential if their effect is to be directed to the substantially selective removal of the saturated aliphatic aldehyde contaminants. The catalysts are effective in exceedingly small amounts. In general, they need not be added in an amount exceeding about 1 M percent of the impure alpha,beta-unsaturated aliphatic aldehyde undergoing treatment. An amount of the aluminum secondary alcoholate in the range of from about 0.1 M percent to about 0.5 M percent, and more preferably in the range of from about 0.15 M percent to about 0.3 M percent, of the impure unsaturated aldehyde being treated will be found satisfactory.

Treatment of the impure, alpha, beta unsaturated aliphatic aledehyde in accordance with the invention is effective at temperatures below about 50 C., preferably at a temperature below about 40 C. At substantially higher temperatures the objects of the invention are not attained. Temperatures as low as, for example, about 25 C. may be employed within the scope of the invention. In general, it is preferred to carry out the treatment at a temperature in the range of from about C. to about 30 C. The treatment is carried out at substantially atmospheric pressure, though subatmospheric pressures and superatmospheric pressures may be employed within the scope of the invention.

It is essential that the feed be dry before being brought into contact with the aluminum alcoholate catalyst. Even exceedingly small amounts of water react selectively with the catalyst and the resulting reaction product is ineffective in obtaining the desired objects of the invention. In view of the fact that the aluminum alcoholate is employed in only catalytic amounts it is essential that water, if present in the charge, be removed therefrom before the treatment. Drying of the alpha,beta-unsaturated aliphatic charge to the process may be carried out in any suitable manner by means disclosed heretofore in the prior art.

The time of contact of aluminum alcoholates with the aldehyde being treated may vary within the scope of the invention. The specific contact time employed will depend to some extent upon the degree to which it is desired to eliminate the contaminating saturated aldehydes. It has been found that a contact time in the range of, for example, from about 0.5 hour to about 5 hours, preferably from about 1 hour to about 3 hours, is satisfactory. Longer or shorter contact times may, however, be employed within the scope of the invention.

Under the above-defined conditions the saturated aliphatic aldehyde will undergo reaction involving selfcondensation, and to some extent reaction with unsaturated aliphatic aldehyde, with the formation of higher boiling reaction products consisting essentially of mixed esters. The treatment of acrolein containing contaminating amounts of propionaldehyde will result in the formation of higher boiling reaction products comprising propyl propionate, allyl propionate and propyl acrylate as the predominating constituents thereof. When acetaldehyde is additionally present the reaction products formed will additionally contain ethyl acetate, allyl acetate and ethyl acrylate. The unsaturated aldehyde is thereafter readily separated from non-volatile materials comprising catalyst and from volatile reaction products by ordinary distillation means.

In carrying out the process of the invention the impure alpha,beta-unsaturated aliphatic aldehyde, for example, acrolein containing propionaldehyde in a contaminating amount, is treated to remove substantially all water therefrom. The dry charge is then cooled to the reaction temperature and the aluminum secondary alkoxide is thereupon added to the cooled charge. The catalyst is permitted to remain in contact with the charge within the above-defined temperature range, for a suflicient length of time to result in the reaction of at least a substantial amount of the propionaldehyde present. In a preferred method of carrying out the invention the mixture is agitated during the treatment, for example, by stirring or other suitable agitating means. The acrolein is then separated from the treated charge.

Separation of acrolein, free of any substantial amount of saturated aldehyde, from the treated mixture may be effected by conventional means comprising one or more such steps, as for example, decantation, stratification, distillation, etc. In a preferred method of carrying out the invention the volatile components of the treated mixture are separated from non-volatile constituents comprising catalyst by evaporation and/or distillation under reduced pressure. The volatilized material, comprising acrolein and higher boiling reaction products formed during the treatment, is then subjected to a distillation to effect the removal of acrolein free of any substantial amounts of propionaldehyde therefrom. Materials other than acrolein such as, for example, ketones introduced with the charge, or esters formed during the treatment, may be removed from the acrolein by conventional means.

The efficiency with which saturated aldehydes are removed from alpha,beta-unsaturated aliphatic aldehydcs in accordance with the invention is evidenced by the following examples:

EXAMPLE 1 In a plurality of separate operations identified herein as runs A, B, C, D, E and F, acrolein containing propionaldehyde and acetaldehyde in contaminating amounts, was brought to reaction temperature. Thereupon there was added to the impure acrolein charge a catalytic amount of aluminum secondary butoxide. The mixture was maintained at the reaction temperature with stirring. Thereupon the volatile components of the mixture were separated from non-volatile catalyst-containing material by distillation under reduced pressure. The volatile components of the treated material thus separated from the non-volatile components were subjected to distillation to effect the recovery of acrolein therefrom. In the following Table I there are indicated the composition of the acrolein charge subjected to the treatment, the amount of aluminum secondary butoxide catalyst employed, reaction temperature and contact time, as well as the results obtained in terms of the composition of the treated acrolein produced and the percent of acrolein recovered, for each of the separate runs.

Table 1 Run A B O D E F Composition of Charge in percent w.:

Acrolein 93. 2 94. 2 94. 0 94. 2 93. 4 93. 6 Propionaldehyde 4. 9 3. 9 4. 1 3. 9 4. 6 4. 4 Aceta1dehyde 0. 7 0.7 0. 7 0. 7 0.9 0.8 Acetone 1.2 1.2 1.2 1.2 1.1 1.2 Amount of Catalyst, percent in.

on Aldehyde Charged 0.14 0 25 0 24 0 30 0.22 0.11 Reaction Temperature, C.-. 20 16 1 18 20 45 Reaction Time, Hours 3 1 3 3 2 1 Composition of Treated Acrolein percent W.:

Acrolein 97. 8 98.0 98.4 98.6 98.4 96. 4 Propionaldehyde 0.8 0.49 0.08 0 .20 1.8 Acetaldehyde 0.04 0.11 0.07 0 .16 .47 Acetone 1.4 1. 4 1.4 1. 4 1. 2 1.3 Acrolein Recovery, percent 90.6 92 89.6 82.7 93 92. 7

EXAMPLE II To acrolein containing 3.7% by weight of propionaldehyde there is added 0.35 mol percent of aluminum ethox ide at a temperature of 20 C. The resulting admixture is stirred for 45 minutes at 20 C. and thereafter subjected to a vacuum distillation to effect "a separation between non-volatile and volatile components. Acrolein having a propionaldehyde content of less than 0.2% by weight is distilled from the separated volatile components of the mixture with an acrolein recovery of 92.5%.

Similarly eflicient removal of propionaldehyde from acrolein is obtained substituting aluminum isoperoxide for the aluminum ethoxide.

The invention claimed is:

1. The process for removing saturated aliphatic aldehydes from an alpha,beta-unsaturated aliphatic aldehyde containing said saturated aldehydes in a contaminating amount, which comprises contacting said saturated aldehyde-containing alpha,beta-unsaturated aldehyde with a catalytic amount of an aluminum alcoholate at a temperature of from about 25 to about 50 C., under substantially anhydrous conditions, and separating said alpha, beta-unsaturated aliphatic aldehyde free of any substantial amount of saturated aldehydes from the resulting mixture.

2. The process in accordance with claim 1 wherein said aluminum alcoholate is an aluminum alcoholate of an alcohol having from 1 to 10 carbon atoms.

3. The process for removing saturated aliphatic aldehydes from an alpha,beta-unsaturated aliphatic aldehyde containing said saturated aldehyde in a contaminating amount, which comprises contacting said saturated aldehyde-containing alpha,beta-unsaturated aldehyde, under substantially anhydrous conditions, at a temperature of from about -25 to about 50 C., with from about 0.1 to about 1 mol percent, based on said unsaturated aldehyde, of an aluminum alcoholate of an alcohol having from 1 to 10 carbon atoms, and distilling said alpha,beta-unsaturated aldehyde free of any substantial amount of saturated aldehydes from the resulting mixture.

4. The process in accordance with claim 3 wherein said aluminum alcoholate is an aluminum alkoxide containing from 3 to 8 carbon atoms in each of the alkoxide groups.

5. The process in accordance with claim 4 wherein said saturated aldehyde-containing unsaturated aldehyde is contacted with said aluminum alkoxide at a temperature of from about 0 to about 30 C.

6 6. The process for removing saturated aliphatic aldehydes from an alpha,beta-unsaturated aliphatic aldehyde selected from the group consisting of acrolein and methacrolein, said saturated aliphatic aldehydes having boiling temperatures closely approximating that of the unsaturated aldehyde containing them, which comprises contacting said saturated aldehyde-containing alpha,-betaunsaturated aldehyde with a catalytic amount of an aluminum alkoxide having from 3 to 8 carbon atoms in each of the alkoxide groups, under substantially anhydrous conditions, at a temperature of from about -25 to about 50 C. and distilling said alpha,beta-unsaturated aliphatic aldehyde free of any substantial amount of saturated aliphatic alcohols from the resulting mixture.

7. The process in accordance with claim 6 wherein said aluminum alkoxide is contacted with said saturated aldehyde-containing alpha,beta-unsaturated aldehyde at a temperature of from about 0 to about 30 C.

8. The process for removing saturated aliphatic aldehydes from an alpha,beta-unsaturated aliphatic aldehyde selected from the group consisting of acrolein and methacrolein, said saturated aldehydes having substantially the same number of carbon atoms as said unsaturated aldehyde containing them, which comprises contacting said saturated aldehyde-containing alpha,betaunsaturated aldehyde with from about 0.1 to about 1 mol percent, based on said unsaturated aldehyde, of an aluminum alkoxide having from 3 to 8 carbon atoms in each of the alkoxide groups, under substantially anhydrous conditions, at a temperature of from about 25 to about 50 C., and distilling said alpha,beta-unsaturated aliphatic aldehyde free of any substantial amount of saturated aldehydes from the resulting mixture.

9. The process for removing propionaldehyde from acrolein, which comprises contacting said propionaldehyde-containing acrolein, under substantially anhydrous conditions, at a temperature of from about 25 to about 50 C., with a catalytic amount of an aluminum alcoholate, arid distilling acrolein free of any substantial amount of propionaldehyde from the resulting mixture.

10. The process in accordance with claim 9 wherein said aluminum alcoholate is an aluminum alcoholate of an alcohol having from 1 to 10 carbon atoms.

11. The process in accordance with claim 10 wherein said aluminum alcoholate is contacted with said propionaldehyde-containing acrolein at a temperature of from about 0 to about 30 C.

12. The process for removing propionaldehyde from acrolein which comprises contacting said propionaldehyde-containing acrolein, under substantially anhydrous conditions, at a temperature of from about -25 to about 50 C., with from about 0.1 to about 1 mol percent, based on said acrolein, of an aluminum secondary alkoxide having from 3 to 8 carbon atoms in each of the alkoxide groups.

13. The process in accordance with claim 12 wherein said aluminum alkoxide is aluminum secondary butoxide.

References Cited in the file of this patent UNITED STATES PATENTS 2,606,933 Cole et al. Aug. 12, 1952 2,774,792 Carlson et a1. Dec. 18, 1956 2,870,221 Hall et a1. Jan. 20, 1959 2,903,491 Reppe et al. Sept. 8, 1959 

3. THE PROCESS FOR REMOVING SATURATED ALIPHATIC ALDEHYDES FROM AN ALPHA,BETA-UNSATURATED ALIPHATIC ALDEHYDE CONTAINING SAID SATURATED ALDEHYDE IN A CONTAMINATING AMOUNT, WHICH COMPRISES CONTACTING SAID SATURATED ALDEHYDE-CONTAINING ALPHA,BETA-UNSATURATED ALDEHYDE, UNDER SUBSTANTIALLY ANHYDROUS CONDITIONS, AT A TEMPERATURE OF FROM ABOUT -25* TO ABOUT 50*C., WITH FROM ABOUT 0.1 TO ABOUT 1 MOL PERCENT, BASED ON SAID UNSATURATED ALDEHYDE, OF AN ALUMINUM ALCOHOLATE OF AN ALCOHOL HAVING FROM 1 TO 10 CARBON ATOMS, AND DISTILLING SAID ALPHA,BETA-UNSATURATED ALDEHYDE FREE OF ANY SUBSTANTIAL AMOUNT OF SATURATED ALDEHYDES FROM THE RESULTING MIXTURE. 